sftp: fix rmdir on Windows based servers (eg CrushFTP)
sftp: fix rmdir on Windows based servers (eg CrushFTP)

Before this change we used Remove to remove directories.  This works
fine on Unix based systems but not so well on Windows based ones.
Swap to using RemoveDirectory instead.

olihey/rclone,ncw/rclone,B4dM4n/rclone,B4dM4n/rclone,ncw/rclone,olihey/rclone,olihey/rclone,olihey/rclone,ncw/rclone,B4dM4n/rclone,B4dM4n/rclone,ncw/rclone